Method of selecting the foot plane angle in a sliding activity

ABSTRACT

In various sliding activities or sports, such as ice skating, a desired angle for the foot plane within a boot is selected. When conditions change, such as going to a different location or even changes within the original location such as temperature changes, the original foot plane angle is no longer appropriate to give the best convenience and performance results. In order to determine the more appropriate foot plane angle the friction coefficient of the surface for which the original foot plane angle existed is determined and then the new friction coefficient is measured at the new location or under the changed conditions. The change in initial friction coefficient to the new friction coefficient is then used to determine what change should be made to the foot plane angle.

BACKGROUND OF INVENTION

In various types of sliding activities or sports, such as ice skating,roller skating and skiing, it is desirable to provide the user withboots wherein the foot plane of the user is in an angular position toprovide comfort and to optimize performance. It is known from, forexample, U.S. Published Application US2013/0062840 to adjust the planeof the skate of the user's foot, such as an ice skater, by placing shimsunder the heel or under the ball of the foot in the boot. All of thedetails of that published application are incorporated herein byreference thereto. U.S. Published Application US2013/0001902 alsodiscloses the use of providing shims to adjust the forward pitch of anice skate. All of the details of that published application are alsoincorporated herein by reference thereto.

Initially the desired foot plane angle would be selected which couldachieve the desired comfort and performance results. However, if thereare changes in conditions, such as performing the activity in adifferent location or under changed conditions in that same location,the original foot plane angle may no longer provide the bestcomfort/performance results.

SUMMARY OF INVENTION

An object of this invention is to provide a method of selecting a footplane angle in a boot used in a sliding activity such as ice skating,roller skating or skiing.

A further object of this invention is to provide such a method whichtakes into account changed conditions and provides guidelines forcorrespondingly changing or adjusting the foot plane angle.

In accordance with this invention the friction coefficient is utilizedas a guideline for determining the best or change in foot plane angle.This is done by first determining the initial friction coefficientbetween the boot contact edge/surface and the surface of the slidingactivity when the initial foot plane angle had been selected. Later whenthere is a change in conditions causing a friction coefficient change,such as moving to a different location or there has been a temperaturechange in the original location, the friction coefficient is againmeasured. If the friction coefficient increases, then the original footplane angle would be decreased. If the friction of coefficient decreasesthen the foot plane angle would be increased.

The change in foot plane angle could be accomplished in any suitablemanner, such as by selection of the proper size shim placed under eitherthe heel or the ball of the foot to thereby elevate or lower the footplane angle to the desired degree.

THE DRAWINGS

FIG. 1 illustrates the position of a person in the non-sliding balancedposition of a balance sport;

FIG. 2 illustrates a person in the unbalanced position of a slidingbalance sport;

FIG. 3 illustrates a person in the sliding balanced position in asliding balanced sport;

FIG. 4 illustrates the forces for a sliding balanced position in arotationally stable sliding balanced sport;

FIG. 5 is a view similar to FIG. 4 of the sliding unbalanced position ina rotationally unstable sliding balanced sport;

FIG. 6 is a view similar to FIG. 5 of the sliding balanced position in arotationally unstable sliding balance sport;

FIG. 7 is a graph correlating the angle adjustment with the change infriction coefficient for a hockey skate in accordance with thisinvention;

FIG. 8 is a graph correlating the angle adjustment with the change infriction coefficient for a speed skating skate in accordance with thisinvention;

FIG. 9 is a graph correlating the angle of adjustment with the change inthe angle of adjustment friction coefficient for a figure skating skatein accordance with this invention;

FIG. 10 is a graph correlating the angle adjustment with the change infriction coefficient (rolling resistance coefficient) for roller skatesin accordance with this invention; and

FIG. 11 is a graph correlating the angle of adjustment with the changein friction coefficient for Nordic skis in accordance with thisinvention.

DETAILED DESCRIPTION

The present invention is based, in part, on the observation that askater might have a good feeling on the ice one day but not another.Such good feeling means that the skater would not be struggling to holdthe technique together and would not be unnecessarily fatigued in doingso. When considering why the feeling might change, the skater's postureand the mechanics of skating are taken into account. The primary conceptof skating is balance. Mechanically speaking there is a balance betweenthe forces and torques acting on skaters. The skater feels a force as apressure which tries to move the skater in one direction or another. Atorque is felt as force that tries to rotate the skater in a direction.A force creates a torque if the line of direction of the force does notgo through the skater's center of gravity (essentially the bellybutton).

Balance is centered about the center of gravity of the skater. When theskater is in the good feeling posture, a conceptual line from the centerof gravity to the intersection point of the blade rocker and the ice iscollinear with the direction of the resultant force (combining thenormal and friction forces). See FIGS. 2 and 3. Thus, the feeling occurswhen the angles between the body's limbs are comfortable and no extraexertion is necessary. When ice friction changes, the direction of thisresultant force will change with respect to the horizontal and thus,will not be collinear with the conceptual line. This will cause atorque. The skater must then adjust his/her posture to a morecomfortable one to eliminate the torque.

It has been observed that only a relatively small foot angle change isneeded, even for a relatively large friction coefficient change withinthe realm of the normal skating ice condition. A skater needs only tochange the lift by 0.2 mm (the thickness of a file folder) to feel asignificant change in the move to or from the good feeling.

Use of shims or lifts even smaller than 0.2 mm are also felt by theuser.

In various types of sliding activities or sports, such as differentforms of ice skating, roller skating or skiing, the good feeling isachieved when foot plane angle is in its optimum position. The footplane angle is determined by the plane at the sole of the foot (from theheel to ball of the foot) within a boot used for that activity. Theangle could be adjusted through the use of shims as described in U.S.Published Application 2013/0062840 or by other methods.

This invention is directed to a method of determining the most efficientand comfortable plane-of-motion foot plane angle, relative to the ice(sliding surface), for ice skates (or any footwear for standing slidingactivity) and then adjusting this angle for changing sliding conditions.

This is accomplished by first determining the initial balance point forthe sliding person and by measuring initial conditions. Then, throughcertain determinations, when initial conditions change, a new foot planeangle is selected.

The invention is based upon the following observations.

With reference to FIG. 1, looking at a person in profile (xy plane), aconceptual line from the center of gravity to the ball of the foot isconsidered (Line_(CoG)) or Center of Gravity Line. When a personcrouches to vertically jump or to pick up an object, on one foot or two,perhaps the most efficient and comfortable maneuver requires that aconceptual Line_(CoG) needs to continuously (through jumping or standingmotion) pass through the same point in each of the: torso, upper legbones, lower leg bones and the foot. The bisect line through the anglesformed between lines connecting these body part joints then needs to beperpendicular to the Line_(CoG). During this maneuver, the minimumextent of Line_(CoG) is normally determined from the acute limit of theangle between the lower leg and the foot plane when the foot is in theactivity-specific footwear. Thus, an angle “α”, as shown in FIG. 1, isdefined as the foot-plane angle relative to the horizontal, when thelength of Line_(CoG) is at a minimum and when the person is thenon-sliding balanced position wearing his/her activity-specificfootwear. While this may be the most efficient and comfortable posture,it is not necessary for use of this invention. Other postures may beconsidered in the same way.

In the Non-sliding Balanced Position case of FIG. 1, the direction ofthe Reaction Force, from the standing surface, passes through the Centerof Gravity, thereby the person is balanced and stable. A shim of angle“a” placed under the heel can relieve the muscle stress in the lower legand foot in the crouched position.

If the sliding person maintained the same body posture (relative to thesurface) as a non-sliding person, the frictional component of thereaction force at the ball of the foot would make the person unbalanced(FIG. 2 Unbalanced Position). This is because the direction of thereaction force would not be aligned with the Line_(CoG), i.e. throughthe person's center of gravity; thus there would be a torque acting onthe body causing the person to topple forward (assuming that airfriction causes no added torque, as it acts approximately through thecenter of gravity).

For a person performing friction sliding (e.g. ice skating, skiing, orroller skating), while either maintaining body posture or trying toexert a force through the ball of the foot (as in a glide, jump or pushrespectively), the most efficient maneuver then would require the personto angularly adjust the Line_(CoG) relative to the sliding surface bypositioning the body to maintain rotational balance without changing therelative position of the Line_(CoG) with respect to the body, i.e. forexample through the points described above (FIG. 1 Non-sliding BalancedPosition); the direction of the reaction force would be through theperson's center of gravity and therefore no torque would be acting onthe body. Another way of regarding this would be for the person toangularly adjust the Line_(CoG), relative to the sliding surface, by anangle β (FIG. 3 Sliding Balanced Position).

The change in the angle α, from the FIG. 2 Unbalanced Position to theFIG. 3 Sliding Balanced Position respectively, demonstrates that thefoot plane angle would need to be reduced by angle β to maintain thebody posture of a balanced person as in FIG. 1. Therefore α_(now)=α−β,where

$\beta = {{\tan^{- 1}\left( \frac{{friction}_{sliding}}{{weight}_{person}} \right)} = {{\tan^{- 1}\left( {{friction}\mspace{14mu} {coefficient}} \right)}.}}$

It is noted that the frictional component of the reaction force could belarge enough that alpha becomes negative and, if using shims to adjustthe foot plane angle, a ball of the foot shim would be needed instead ofa heel shim.

While this formula works well for when the contact surfaces are straightor are rotationally stable in the xy plane, it must be modified when oneof the surfaces is curvilinear, such as an ice skate, which is unstablein the xy plane. This is demonstrated in the FIGS. 4-6 RotationallyStable—Sliding Balanced Position, Rotationally Unstable—SlidingUnbalanced Position, and Rotationally Unstable—Sliding Balanced Positionwhere the contact areas of the sliding person and the surface aremagnified.

When one of the contact surfaces is curvilinear, such as an ice skate,the angle α_(new) must not only take into account the frictional force,but also the rotation of the skate that is necessary to eliminate thetorque caused by the misalignment of the Reaction Force from theLine_(CoG), as shown in the FIG. 5 Rotationally Unstable—SlidingUnbalanced Position. Because the curvilinear contact surface will needto adjust to align the Reaction Force with the Line_(CoG), it will needto further rotate an angle of β. Therefore the α_(new)=α−2β (FIG. 6Rotationally Unstable Sliding Balanced Position), which takes intoaccount the angle necessary to address friction and instability.

The following conclusions are made.

Conclusion 1

To maintain efficient rotational balance in a sliding sport, thatLine_(CoG) needs to adjust from a balanced standing position (a verticalline) to a balanced sliding position by a specific angle.

Conclusion 2

For a sliding sport in which the sliding contact area is straight in thexy plane or has at least two contact points that are on the xy plane(for example roller skates), the angle of that Line_(CoG) is equal tothe inverse tangent of the coefficient of friction between the contactmaterials.

Conclusion 3

For a sliding sport in which the sliding contact area is curved (forexample ice skates), the angle of Line_(CoG) is equal to twice theinverse tangent of the coefficient of friction between the contactmaterials.

Conclusion 4

For a sliding sport in which the sliding contact area is straight, alift, in either under the heel or under the ball of the foot to attainthe specific angle, is approximately or essentially equal to thedistance between the heel and the ball of the foot multiplied by the sumof the coefficient of friction between the contact materials and theangle “α”.

Conclusion 5

For a sliding sport in which the sliding contact area is curved, a lift,in either under the heel or under the ball of the foot to attain thespecific angle, is approximately or essentially equal to the distancebetween the heel and the ball of the foot multiplied by the twice thesum of the coefficient of friction between the contact materials and theangle “α”.

Conclusion 6

By measuring the friction, either directly through a friction measuringdevice (such as a tribometer) or indirectly by measuring a dependentvariable (for example temperature, contact area, etc.), a definite footplane angle or size lift, for either under the heel or under the ball ofthe foot, for sliding person's stability can be attained.

Conclusion 7

Any imbalance in a sliding person, caused by a change in friction, canbe corrected by using shims or changing the angle of the foot plane by aspecified amount as calculated by Conclusions 3 through 6.

When there is a change in conditions, namely a change in the surfacecontact friction, a new foot plane angle may be determined usingformulas in the following manner.

-   -   1) Determine initial conditions of: contact surface friction        (either directly from a tribometer or by determination from        dependent variables, e.g. temperature, geometry, etc.), sliding        person's weight, and efficient balanced posture (non-sliding).    -   2) Adjust the foot plane angle by methods of Conclusions 1)        through 6) or otherwise until gliding position is stable    -   3) When sliding surface conditions change, adjust foot plane        angle for:        -   i. a stable sliding contact surface in the plane of motion            by the angle calculated by            tan⁻¹(μ_(initial))−tan⁻¹(μ_(changed)) where μ is the contact            friction OR        -   ii. an unstable sliding contact surface in the plane of            motion by the angle calculated by            2×[tan⁻¹(μ_(initial))−tan⁻¹(μ_(changed))]            -   OR

When sliding surface conditions change, add heel or ball of the footshims:

-   -   i) a stable sliding contact surface in the plane of motion by        the amount calculated by L        tan[tan⁻¹(μ_(initial))−tan⁻¹(μ_(changed))] where L is the heel        to ball of the foot length OR    -   ii) an unstable sliding contact surface in the plane of motion        by the amount calculated by L        tan{2×[tan⁻¹(μ_(initial))−tan⁻¹(μ_(changed))]}

The above four formulas may be stated in words as follows:

(a) The inverse tangent of the initial friction coefficient minus theinverse tangent of the new friction coefficient;(b) Two times the difference between the inverse tangent of the initialfriction coefficient and the inverse tangent of the new frictioncoefficient;(c) The heel to ball of the foot length times the tangent of thedifference between the inverse tangent of the initial frictioncoefficient and the inverse tangent of the new friction coefficient;(d) The heel to ball of the foot length times the tangent of 2 times thedifference between the inverse tangent of the initial coefficient andthe inverse tangent of the new friction coefficient.

As is apparent steps 1) and 2) above relate to establishing the initialor original foot plane angle which gives the person the best positionfor the initial friction coefficient. When there has been a change infriction coefficient, such as from being in a different location orchanged conditions (e.g. temperature) in the same location, step 3) isused for determining the new foot plane angle.

In general, all ice skates, because of the rocker will have the sameslope on a friction vs. foot plane angle graph. Skis and roller skateswill have a different slope.

Subjectively an average person can feel a change of 0.2 mm lift, whichequates to approximately a 0.0644 degree (64.4×10⁻³ degrees) changewhich is a change in friction coefficient of 0.56×10⁻³ (i.e. 0.0644divided by −115).

The invention provides for the determination of the foot plane anglegiven the friction. The invention also provides practical guidelines fordetermining what change should be made to obtain a new foot plane anglewhere there has been a change in friction coefficient. FIGS. 7-11 aregraphs correlating changes in friction coefficient with change in footplane angle for different activities. The graphs are accurately to scalefor purposes of this invention. In general when there has been anincrease in friction coefficient, there will be a decrease in foot planeangle as determined by the appropriate graph. Conversely, when there hasbeen a decrease in friction coefficient, the foot plane angle willincrease by the corresponding amount on the graph. The change in footplane angle can be achieved by use of shims in the boot at the heel orat ball of the foot. Thus, for example, when it is intended to increasethe angle, a shim would raise the heel.

FIGS. 7-11 are graphs for specific activities. In each graph, thenumbers along the horizontal axis, as stated at the bottom of thevertical axis, are the changes in friction coefficient. In practice theinitial friction coefficient and the new friction coefficient would bedetermined and their difference would be a value on the horizontal axis.The change needed for the new foot plane angle would be thecorresponding amount on the vertical axis at the corresponding point onthe appropriate line in the graph for the particular activity. The sameprocedure would be repeated when there is a later further change inconditions wherein the prior friction coefficient would be considered asthe initial friction coefficient.

FIG. 7 is a graph for a hockey skate and is referred to as The HockeySkate Graph.

For all intents and purposes, between the friction coefficient valuesfound in hockey skating, the curve is a straight line with a slope ofapproximately −115 degrees. In other words a positive change in thecoefficient of friction of 1 would yield a negative change of 115degrees in the foot plane angle (i.e. by raising the ball-of-foot liftand/or lowering the heel lift, or any other means of changing theangle).

Thus, in The Hockey Skate Graph the friction coefficient ranges would befrom 0.00677 to 0.01488 (another way of expressing these values is inscientific form, i.e. 6.77×10⁻³ to 14.88×10⁻³). The corresponding anglechange would be less than 1 degree.

FIG. 8 is a graph for speed skating skates and is referred to as TheSpeed Skating Graph. It is noted that values on the horizontal axis, forconvenience, are referred to by their exponential value. Thus, forexample, 5.00 E-03 refers to 0.005 since E-03 means three decimalpoints. Similarly, 1.00 E-02 refers to 0.01, since E-02 refers to twodecimal points.

For all intents and purposes, between the friction coefficient valuesfound in speed skating, this curve is a straight line with a slope ofapproximately −115 degrees. In other words a positive change in thecoefficient of friction of 1 would yield a negative change of 115degrees in the foot plane angle (i.e. by raising the ball of foot liftand/or lowering the heel lift, or any other means of changing theangle).

Thus, the friction coefficient ranges in The Speed Skating Graph wouldbe from 0.00282 to 0.01008 (another way of expressing these values is inscientific form, i.e. 2.82×10⁻³ to 10.08×10⁻³). The corresponding changein the foot plane angle would be no greater than 0.8 degrees.

FIG. 9 is a graph for figure skating skates and is referred to as TheFigure Skating Graph.

For all intents and purposes, between the friction coefficient valuesfound in figure skating, this curve is a straight line with a slope ofapproximately −115 degrees. In other words a positive change in thecoefficient of friction of 1 would yield a negative change of 115degrees in the foot plane angle (i.e. by raising the ball of foot liftand/or lowering the heel lift, or any other means of changing theangle).

Thus, the friction coefficient ranges in The Figure Skating Graph wouldbe from 0.01373 to 0.04070 (another way of expressing these values is inscientific form, i.e. 13.73×10⁻³ to 40.70×10⁻³). The correspondingchange in foot plane angle would be less than 2.5 degrees.

FIG. 10 is a graph for roller skating skates and is referred to as TheRoller Skating Graph. For purposes of this invention, the rollingresistance coefficient which is applicable to roller skates, is treatedas friction coefficient.

For all intents and purposes, between the rolling resistance coefficientvalues found in in-line roller skating, this curve is a straight linewith a slope of approximately −57 degrees. In other words a positivechange in the coefficient of friction of 1 would yield a negative changeof 57 degrees in the foot plane angle (i.e. by raising the ball of footlift and/or lowering the heel lift, or any other means of changing theangle).

Thus, the friction coefficient ranges in The Roller Skating Graph wouldbe from 0.04 to 0.075 (another way of expressing these values is inscientific form, i.e. 40×10⁻³ to 75×10⁻³). The corresponding change infoot plane angle is no greater than 2 degrees and more specifically isbetween 1 and 2 degrees.

FIG. 11 is a graph for Nordic Skis and is referred to as The Ski Graph.

For all intents and purposes, between the friction coefficient valuesfound in Nordic skiing, this curve is a straight line with a slope ofapproximately −57 degrees. In other words a positive change in thecoefficient of friction of 1 would yield a negative change of 57 degreesin the foot plane angle (i.e. by raising the ball of foot lift and/orlowering the heel lift, or any other means of changing the angle).

Thus, the friction coefficient ranges in The Ski Graph would be from0.02 to 0.06 (another way of expressing these values is in scientificform, i.e. 20×10⁻³ to 60×10⁻³). The corresponding change in foot planeangle is less than 2.5 degrees.

The present invention is thus based upon taking into account that thereshould be a change in foot plane angle when there is a change infriction coefficient in a sliding activity, such as ice skating, rollerskating and skiing.

When the change in friction coefficient is determined, an appropriatechange in foot plane angle can be made.

What is claimed is:
 1. In a method of selecting the angle of a footplane of a foot in the boot used in a sliding activity wherein the footplane is originally at an original angle for a surface having an initialfriction coefficient, the improvement being in changing the foot planeangle when the friction coefficient between the boot contactedge/surface and the surface of the sliding activity changes, comprisingthe steps of determining the initial friction coefficient, latermeasuring a new friction coefficient for the surface of a slidingactivity, decreasing the foot plane angle from the original angle to anew angle when the friction coefficient increases, and increasing thefoot plane angle to a new angle when the friction coefficient decreases.2. The method of claim 1 wherein the new foot plane angle is obtained bythe use of a shim inserted in the boot at the heel or at the ball of thefoot.
 3. The method of claim 1 wherein the boot is a hockey skate boot,and when the friction coefficient changes in the range of 0.00677 to0.01488 the foot plane angle is changed by an angle in the range of lessthan 1 degree.
 4. The method of claim 1 wherein the boot is a hockeyskate boot and the foot plane angle is changed in accordance with thechange in friction coefficient on The Hockey Skate Graph.
 5. The methodof claim 1 wherein the boot is a speed skating boot, and when thefriction coefficient changes in the range of 0.00282 to 0.01008 the footplane angle is changed in a range no greater than 0.8 degrees.
 6. Themethod of claim 1 wherein the boot is a speed skating boot and the footplane angle is changed an amount based on the friction coefficientchange in The Speed Skating Graph.
 7. The method of claim 1 wherein theboot is a figure skating boot and when the friction coefficient changesin the range of 0.01373 to 0.04070 the foot plane angle is changed in arange of less than 2.5 degrees.
 8. The method of claim 1 wherein theboot is a figure skating boot and the foot plane angle is changed by anamount corresponding to the friction coefficient change in The FigureSkating Graph.
 9. The method of claim 1 wherein the boot is a rollerskating boot and when the friction coefficient changes in a range offrom 0.04 to 0.075 the foot plane angle changes in a range no greaterthan 2 degrees.
 10. The method of claim 1 wherein the boot is a rollerskating boot and the foot plane angle changes by an amount correspondingto the change in friction coefficient in The Roller Skating Graph. 11.The method of claim 2 wherein the boot is a ski boot and when thefriction coefficient changes in the range of from 0.02 to 0.06 the footplane angle changes in a range of less than 2.5 degrees.
 12. The methodof claim 1 wherein the boot is a ski boot and the foot plane angle ischanged by an amount corresponding to the change in friction coefficientin The Ski Graph.
 13. The method of claim 1 wherein a tribometer is usedto measure the friction coefficient.
 14. The method of claim 1 whereinfriction is determined based on temperature measurement.
 15. The methodof claim 1 wherein the foot plane angle is selected to maintain areaction force at the surface of the sliding activity aligned with thecenter of gravity line of the person in the sliding activity.
 16. Themethod of claim 1 wherein the foot plane angle change for a stablesliding contact surface is calculated by the inverse tangent of theinitial friction coefficient minus the inverse tangent of the newfriction coefficient and for an unstable sliding contact surface thefoot plane angle is calculated by 2 times the difference between theinverse tangent of the initial friction coefficient and the inversetangent of the new friction coefficient.
 17. The method of claim 1wherein the foot plane angle is changed by the addition of a shim to theheel or to the ball of the foot wherein for a stable sliding contactsurface the angle is calculated by the heel to the ball of the footlength times the tangent of the difference between the inverse tangentof the initial friction coefficient and the inverse tangent of the newfriction coefficient, and for an unstable sliding contact surface thefoot plane angle is calculated by the heel to the ball of foot lengthtimes the tangent of 2 times the difference between the inverse tangentof the initial friction coefficient and the inverse tangent of the newfriction coefficient.
 18. The method of claim 1 wherein the method isrepeated when there is a subsequent change in friction coefficientwhereby the previous new friction coefficient is treated as the initialfriction coefficient and the previous new foot plane angle is treated asthe original foot plane angle.
 19. A method of selecting the angle of afoot plane of a foot in the boot used in a sliding activity comprisingof obtaining a balanced sliding position by measuring the coefficient offriction between the boot contact edge/surface and the surface of thesliding activity, and using the coefficient of friction to determine thefoot plane angle by a technique selected from the group consisting of:a) for a sliding sport in which the sliding contact area is straight inthe xy plane or has at least two contact points that are on the xyplane, the angle of foot plane is equal to the inverse tangent of thesum of the coefficient of friction between the contact materials and thefoot-plane angle relative to the horizontal, when the length of theconceptual line from the center of gravity to the ball of the foot(where the conceptual bisect lines through the angles formed between theconnecting body parts torso, upper leg, lower leg and foot areperpendicular to that line) is at a minimum and when the person is thenon-sliding balanced position wearing his/her activity-specificfootwear; b) for a sliding sport in which the sliding contact area iscurved, the angle of the foot-plane is equal to twice the inversetangent of the sum of the coefficient of friction between the contactmaterials and the foot-plane angle relative to the horizontal, when thelength of the conceptual line from the center of gravity to the ball ofthe foot (where the conceptual bisect lines through the angles formedbetween the connecting body parts torso, upper leg, lower leg and footare perpendicular to that line) is at a minimum and when the person isthe non-sliding balanced position wearing his/her activity-specificfootwear, c) for a sliding sport in which the sliding contact area isstraight, a lift, in either under the heel or under the ball of the footto attain the angle, is generally equal to the distance between the heeland the ball of the foot multiplied by the sum of the coefficient offriction between the contact materials and the foot-plane angle relativeto the horizontal, when the length of the conceptual line from thecenter of gravity to the ball of the foot (where the conceptual bisectlines through the angles formed between the connecting body parts torso,upper leg, lower leg and foot are perpendicular to that line) is at aminimum and when the person is the non-sliding balanced position wearinghis/her activity-specific footwear; and d) for a sliding sport in whichthe sliding contact area is curved, a lift, in either under the heel orunder the ball of the foot to attain the angle, is generally equal tothe distance between the heel and the ball of the foot multiplied by thetwice the sum of the coefficient of friction between the contactmaterials and the foot-plane angle relative to the horizontal, when thelength of the conceptual line from the center of gravity to the ball ofthe foot (where the conceptual bisect lines through the angles formedbetween the connecting body parts torso, upper leg, lower leg and footare perpendicular to that line) is at a minimum and when the person isthe non-sliding balanced position wearing his/her activity-specificfootwear.